The cookies are not compatible with each other, plus the third-party cookie can also be named differently (via cookie_name).

Replying to vipsoft:

The cookies are not compatible with each other
1st and 3rd party cookies are not compatible? Why is there a use_third_party_id_cookie flag then?
plus the third-party cookie can also be named differently (via cookie_name).
I kept the default value
cookie_name = piwik_visitor

piwik.js uses a first-party, client-side generated cookie. It doesn't understand the format of the third-party, server-side generated cookie.

Ok, thanks I got it, so it's a normal behaviour then: I thought that getVisitorId() was making an Ajax call, instead of just reading first-party cookie.
However, when I clear recent history, both cookies are deleted and piwik still records the visit under the previous idvisitor, by matching config/IP. So the question is how can I get the real idvisitor value (under which the visit will be stored)? I mean is it possible to get it from the server with Ajax?

No, retrieving the visitor ID by AJAX isn't possible, and I'm not sure if it's planned given the security considerations and bulk loading contemplated in #134.

Hmm, ok. Now that I think about it, I suppose it is always possible to select recent rows inside piwik_log_visit table, and then filter by IP to retrieve idvisitor value with more or less success... Anyway thanks for the explanations and a prompt response.

Just to clarify the default use case (no 3rd party cookies): When you refresh the page with no cookie, Piwik will generate a new visitor ID on the client side (1st party cookie), since you don't have any cookie. Then it will send this cookie value to the server and the log_visit row for this visit should be updated with the newly created idvisitor.

It's still valid in 2.3.0

So never use "use_third_party_id_cookie" setting, when u are tracking also in your backend (e.g. PHP) Otherwise your user will be counted twice.

Solution / Idea?

• read from piwik.js the 3rd party cookie and write it from the inline js to the 1st party cookie? (is this possible?)